For many years, oxide isolation regions have been used to electrically isolate adjacent active devices formed on a semiconductor substrate. These structures can be fully recessed ("ROX"), wherein the upper surface of the oxide region is substantially planar with the upper surface of the substrate; or semi-recessed, wherein the upper surface of the oxide region extends above the upper surface of the substrate.
One common method of forming these regions includes the steps of forming a silicon oxide layer on the substrate, forming a silicon nitride layer on the oxide, and patterning these two layers in order to form an oxygen-impervious "pad". The pad is formed in an area of the substrate where the active devices will be formed. When the substrate is subsequently oxidized, the portions of the substrate left unprotected by the pad oxidize much faster than does the pad itself, thus forming oxide isolation regions between areas of the substrate where the active devices will be formed.
A problem with the above method of forming oxide isolation regions is the formation of the so-called "bird's beak structures". While the unprotected regions of the substrate are oxidized, oxygen impurities (generally referred to as "oxidizing specie") laterally diffuse through the silicon oxide layer of the pad into the protected portions of the substrate. This diffusion causes the sidewalls of the oxide isolation regions to slope inwardly into the protected substrate portions, such that the sidewall profiles resemble a bird's beak.
Several methods have been proposed for eliminating or reducing the formation of these bird's beak profiles. In an article by Ho et al ("Elimination of Bird's Beak in ROI Process", IBM Technical Disclosure Bulletin, Vol. 24, No. 2, July 1981, pp. 1290-1291) a method of reducing bird's beak is disclosed in which the pad is made up of silicon oxide, silicon nitride and silicon oxide layers. The sides of the pad are protected by a layer of silicon nitride. Thus, when the surrounding substrate is oxidized, the silicon nitride on the sidewalls of the pad greatly reduces the lateral diffusion of oxidizing specie through the SiO.sub.2 layer of the pad. The idea of using silicon nitride as a protective coating on the sidewalls of a pad is also shown in Chui et al, "A Bird's Beak Free Local Oxidation Technology Feasible for VLSI Circuits Fabrication", IEEE Transactions on Electron Devices, Vol. ED-29, No. 4, April, 1982, pp. 536-540.
While these methods do result in a reduction of bird's beak profiles, they present several difficulties. In general, silicon nitride produces film stresses when it is deposited on a silicon substrate and subjected to heating. These stresses (in order of 1.times.10.sup.10 dynes/cm.sup.2) are large enough to disturb the crystal lattice of the underlying silicon. In both the Ho and Chui articles, the silicon nitride which is formed on the sidewalls of the pad abuts the silicon substrate, subjecting the substrate to these high film stresses.
Accordingly, it would be advantageous to construct an oxygen-impervious pad in which the pad sidewalls are coated with a material which prohibits lateral diffusion of oxygen while not subjecting the underlying substrate to high film stresses.